1. Field of the Invention
The present invention relates to an optical fiber coil and a manufacturing method thereof used for a chromatic dispersion compensator, a mode dispersion compensator, an optical amplifier, an optical fiber gyroscope and the like.
2. Related Background Art
As an optical fiber coil and a manufacturing method thereof used for an optical amplifier, a chromatic dispersion compensator, a mode dispersion compensator, an optical fiber gyroscope and the like, an optical fiber coil and a manufacturing method thereof described in Japanese Laid-open Patent Publication No. 123342/1998 and the like have been known. The optical fiber coil performs a desired action to optical signals on an optical path. For example, an optical fiber coil used in the optical amplifier is formed by making an EDF (Erbium-Doped optical-Fiber) in a coil shape to amplify the optical signals on the optical path of the optical fiber.
Here, to amplify the light, it becomes necessary for the EDF to ensure some length and it is desirable to form the EDF into a coil bundle to store the EDF spacesavingly in the optical amplifier. To this end, an optical fiber coil which forms the optical fiber formed into a coil bundle has been used. The same goes for optical fiber coils used in other optical parts such as a chromatic dispersion compensator, a mode dispersion compensator, an optical fiber gyroscope and the like besides the optical amplifier. In general, the conventional optical fiber coil has been constituted by winding the optical fiber around a bobbin.
However, the tension remains in the optical fiber which is wound around the bobbin many times and this tension gives rise to the occurrence of microbend loss. Further, due to the difference of linear expansion coefficient between the bobbin and the optical fiber, a stress caused by the deformation of the bobbin is applied to the optical fiber so that the transmission loss is changed depending on temperature. Accordingly, the studies in which the various ideas including the idea described in the above-mentioned publications are made so as to provide a bobbinless optical fiber coil or a bobbin structure having an equivalent effect as the bobbinless optical fiber coil have been made.
However, even with such various ideas, the microbend loss which occurs due to the minute bending of the optical fiber cannot be removed completely. Inventors of the present invention have made extensive studies for minimizing the microbend loss of the optical fiber coil. As a result, the inventors have found that, usually, a resin coating layer formed around a glass portion (core and cladding) of the optical fiber is relevant to the occurrence of the microbend loss. The present invention has been made bade on such a finding and it is an object of the present invention to provide an optical fiber coil which can suppress the microbend loss and exhibit the stable transmission characteristics and a method of manufacturing such an optical fiber coil.
The optical fiber coil according to the present invention is provided with an optical fiber whose core and cladding are made of a glass portion, a storing case which stores the optical fiber wound around into a coil bundle, and a filler filled in the inside of the storing case. Further, the filler is stored in the storing case in the state that the filler directly comes into contact with the glass portion or in the state that the filler directly comes into contact with a thin film coating having a thickness of not more than 1 xcexcm which is formed on the surface of the glass portion and has a hydrogen intrusion suppressing function.
Due to such a constitution, according to the optical fiber coil of the present invention, since it is unnecessary to provide a bobbin for maintaining the coil-shaped state, that is, the coil-shaped state is maintained by using the filler around the optical fiber, the occurrence of the microbend loss can be suppressed and the transmission characteristics can be made stable. Further, the optical fiber of the present invention is not provided with a resin coating layer which an optical fiber usually includes and the filler is filled such that the filler directly comes into contact with the glass portion (including a case in which a thin film coating of not more than 1 xcexcm having a hydrogen intrusion suppressing function is formed on the surface of the glass portion) and hence, the occurrence of the microbend loss can be further suppressed thus further enhancing the stability of the transmission characteristics.
Here, it is preferable that, at a given wavelength within the operating wavelength band of the optical fiber, at least one of the chromatic dispersion or the chromatic dispersion slope of the optical fiber has a sign inverse to a sign of those of an optical fiber for transmission which is optically connected to the optical fiber coil. Further, it is preferable that the operating wavelength band is not less than 1.50 xcexcm. Still further, it is preferable that, at a given wavelength within the operating wavelength band of the optical fiber, the microbend loss of the optical fiber at the time of bending the optical fiber to a radius of curvature of 20 mm is not less than 1 dB/m.
Further, it is preferable that the diameter of the cladding of the optical fiber is not more than 100 xcexcm. Still further, the filler is a material having an undisturbed penetration of JIS K 2220 which falls within a range of 5-200 at a measuring temperature of from xe2x88x9240xc2x0 C. to 100xc2x0 C. The undisturbed penetration is defined by JIS K 2220-1993 of Japanese Industrial Standard [JIS K 2220-1993 bis 2.(14), 5.3.1(4), 5.3.6 and the like]. The measuring temperature is set to 25xc2x0 C. in JIS K 2220. It is preferable. And more, it is more preferable that the material which has the undisturbed penetration which falls within the above mentioned range in the whole range of measuring temperature of xe2x88x9240xc2x0 C. to 100xc2x0 C. is used.
Further, it is preferable that the filler is a material whose hydrogen generation quantity after a temperature degradation test for 24 hours at a temperature of 60xc2x0 C. is not more than 1.0 xcexcl/g. The hydrogen generation quantity is measured as follows. The filler of 1 g which is hardened in a module configuration is sampled and then is put into a glass bottle of 100 ml for a gas chromatography and then 0.04 ml of He (helium) is filled for correction. The glass bottle is held in this condition for 24 hours at 60xc2x0 C. and then the condition is returned to the normal temperature (23xc2x0 C.) and thereafter the measurement using the gas chromatography is performed. The data on the hydrogen generation quantity is arranged using the peak area ratio between He and H2.
Further, it is preferable that the refractive index of the filler is greater than the refractive index of the cladding. Still further, it is preferable that the filler contains the hydrogen absorption material.
Further, the method for manufacturing optical fiber of the present invention includes a coiling step which forms an optical fiber having a resin coating layer on the periphery of a glass portion comprised of a core and a cladding into a coil bundle, a coating layer removing step which removes the resin coating layer from the optical fiber formed into a coil bundle, a storing step which stores the optical fiber from which the resin coating layer is removed in a storing case, and a filling step which fills a filler in the inside of the storing case.
Due to such a constitution, according to the method for manufacturing optical fiber coil of the present invention, since the filler is filled in the storing case such that the filler directly comes into contact with the glass portion after the resin coating layer which the optical fiber usually has is removed, the occurrence of a microbend loss derived from the resin coating layer can be suppressed whereby an optical fiber coil having stable transmission characteristics can be manufactured.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.